Discuss the British policy towards Nepal and Bhutan during 18th and 19th century. To what extent, this policy led Nepal and Bhutan to remain separate nations?

British followed the policy of maintaing Buffer countries between two large nations for security reasons. The same policy was followed in the Himalayan regions of Nepal and Bhutan to maintain security of the northern frontier from its neighbors China and Russia. 
British policy towards Bhutan:
Bhutan had come under the British control after Duar war in1865. In 1910, the British had signed the Treaty of Punakha with Bhutan and this laid the foundation of the relation between India and Bhutan in subsequent times. Through this treaty, the British got the power to regulate Bhutan’s foreign relations and Bhutan was also assured of security against any possible threat from China, its northern neighbor. After the British had left the subcontinent, the relationship between India and Bhutan saw a continuity of this pattern. The Bhutanese agent in India continued to function as before while political representative from India based in Sikkim contributed in looking after Bhutan.
British policy towards Nepal:
The geo-strategic location of Nepal between India and China has shaped British policy towards Nepal. The presence of imperialist China and Russia in the north led British India to consider Nepal has a buffer state. British policy towards Nepal was guided by two treaties, they are

• Treaty of Sugauli was signed between British East India Company and Nepal in 1816
• In 1923, Nepal–Britain Treaty was signed.It was historically significant as it made Nepal an independent and sovereign country. The treaty was recorded in the League of Nations.  It has recommendations like deciding boundary line of Nepal; Nepal ceded territories to British and allowed a British representative in Kathmandu;

This Nepal-Britain treaty led to remain Nepal as a separate nation.